In recent years, projection televisions are spreading as a means for producing an image on a large display screen. In attaining the high quality of the image, the performance of the projection lens system plays an important role.
The projection lens system for a projection television needs to have a large aperture ratio (with an F-number close to 1) to secure the brightness. In addition, the projection lens system must be capable of good correction of aberration to a periphery of a projected image on the display screen.
A problem associated with the projection television is its depth, which is larger than the direct-viewing television (ordinary a television set with the cathode ray tube fluorescent surface is seen directly), so a projection lens system with a shorter projection length is desired.
To achieve high image-forming performance, a large aperture ratio, and a large field angle with a conventional projection lens system formed solely of glass lens elements, the number of lens elements and the aperture of the lens system must be increased. The cost of the lens system is therefore increased. Moreover, the weight of the entire lens system is increased.
Furthermore, correction of the performance in the periphery of the screen is difficult. If aspheric plastic lens elements are employed, correction of the aberration in the periphery is facilitated. But if the aspherical amount of the plastic lens element is increased to increase the corrective power, the precision in the fabrication is lowered, and the design performance is not fully realized. In particular, when the aspheric lens is provided near the screen, because of the large light beam diameter, the precision error easily cause aberration, and due to the occurrence of the glare, the contrast is lowered.